Hmm. The key is that they all need to be also useful enough that you don't throw them out by mistake.

I can just see me putting my coin safe in a parking meter. Or it is part of the kid's allowance. I can see putting the log on the fire. Or getting rid of that book on the History of the Peleponesian Wars. Or using up the foam in the spraycan, and dropping it in the trash.

There are only a couple here that are "safe" - meaning someone won't toss them out, along with your cash. The sprinkler head is one, and the electrical outlet (if you install a special outlet box that is, like the tardis, :"bigger on the inside". Those are available at your local building supply, and can be rigged so that even if someone removes the plate and outlet, your hidden items are still not visible.

This is a cool idea but be sure you leave a note in your safe or in a sealed envelope with a relative disclosing your hidden in plain view "stash". When my secret pot grew to $5k, I started thinking what would happen to it if I didn't come home one day? Could easily be thrown away!